The invention is based on the priority application EP07290738.9 which is hereby incorporated by reference.
The invention relates to a method of performing multicast admission control in a communications network, to a central admission controller for performing multicast admission control in a communications network, and to a communications network comprising such a central admission controller.
For illustrating current practice of multicast admission control, a simplified representation of a portion of a communications network 1 comprising a multicast tree 2 is shown in FIG. 1a. The multicast tree 2 has a source 3 for providing a plurality of services, e.g. voice over IP, Broadcast TV, or Video on Demand (VoD), over a first and second multicast channel 4a, 4b (represented as a sequence of arrows) to a plurality of host systems, a first and second of which 5a, 5b are shown in FIG. 1a. The first multicast channel 4a is established between the source 3 and the first host system 5a by replicating the first multicast channel 4a at each of three replication nodes 1-1 to 3-1 connecting the source 3 to the first host system 5a over corresponding interconnecting links 1.1 to 3.1, T1. The first multicast channel 4a is also established on two further nodes 1-2, 2-2 being connected to the second and third nodes 2-1 and 3-1, respectively. The second multicast channel 4b is established between the source 3, the third node 3-1 and the second of the further nodes 2-2.
For establishing the second multicast channel 4b as a new multicast channel between the second host system 5b and the source 3, the second host system 5b generates a request 6 for a multicast channel which is transmitted to the first node 1-1 in the multicast tree 2 over a corresponding interconnecting link T2, the request being e.g. of the IGMP (Internet Group Management Protocol) type.
As the requested channel is not yet available at the first node 1-1, the first node 1-1 performs Local Admission Control, i.e. it checks local resources, and typically, the downstream interconnecting link T2 to the second host system 5b. If the Local Admission Control at the first node 1-1 succeeds, the first node 1-1 forwards the request 6 for a new multicast channel to the second node 2-1, being the upper node in the multicast tree 2. The same procedure is repeated by the second node 2-1 and the third node 3-1.
The problem with the algorithm illustrated in FIG. 1a is that the total time it takes is a function of the number of IGMP hops from multicast node to multicast node in the multicast tree. As the number of hops increases, the time to perform the Local Admission Control on all replication nodes in the multicast tree also increases. Depending on the business success of Multicast Traffic (such as Broadcast TV), the number of multicast channels and the number of end users may increase in the near future. As a consequence, the probability that a given channel is available at a given node in the multicast tree will decrease. In other terms, the number of IGMP hops on which Local Admission Control needs to be performed will increase, thus the total time to perform the Multicast Admission Control will also increase.
Furthermore, in the algorithm illustrated in FIG. 1a, in case an upper node in the multicast tree 2 is unable to add the new multicast channel, the request 6 for a new multicast channel anyway has to cross several hops before the request is eventually denied, as illustrated in FIG. 1b: the Local Admission Control fails at the third node 3-1 (see the cross in FIG. 1b), but the request 6 nevertheless needs to cross the first node 1-1 and the second node 2-1 and Local Admission Control needs to be performed on all of the three nodes 1-1 to 3-1.
From US 2007/0081459 A1, a module for negotiating multi-service admission control is known which distributes the admission process for multicast channels between a centralized decision function and a distributed, i.e. local admission function. The local admission function can be established in replication nodes of a communications network, allowing the multicast replication nodes to participate in the admission control process.